In the past, it has been desirable to provide some means to protect magnetic recording surfaces and give such surfaces a longer useful life. To accomplish this, hard particles were added to the magnetic recording composition or in the form of a finish coating to be applied to a magnetic recording surface to provide abrasion resistance.
The hard particles which were to provide the abrasion resistance had to be of a material which would not interfere with the magnetic qualities of the magnetic recording surface. The materials chosen would have to be non-magnetizable.
The method of applying the hard particles to the magnetic recording surface proved to be a major obstacle. The method used had to provide an abrasive resistant coating which would not damage the magnetic head. To accomplish this it was determined that the hard particles had to be of a uniform size after dispersing to prevent such damage to the magnetic head. If the hard particles were not uniform after dispersing there would be significant uneven magnetic surface wear and possible magnetic head damage.
There have been different methods in the prior art to provide either an abrasion resistant magnetic recording surface or finish coating. The prior methods have not solved the problem of providing an abrasion resistant magnetic recording surface or finish coating in which the hard particles before and after dispersing are substantially the same size, i.e. no size reduction.
For example, U.S. Pat. No. 3,843,404, the teachings of which are incorporated herein by reference, shows milling hard particles and magnetic particles together to form an abrasion resistant magnetic recording surface. However, this method results in a reduction in the size of the hard particles by an unpredictable amount. This reduction results in an abrasion resistant magnetic recording surface having hard particles of non-uniform size creating a non-uniform surface, resulting in the above-mentioned problems.
As indicated in the foregoing, the prior art contemplates different methods of dispersing the hard particles. The milling method which has been primarily used to disperse hard particles for abrasion resistant surfaces has been ball milling, as shown in U.S. Pat. Nos. 3,080,319 and 3,687,725. Ball milling disperses the hard particles, but it also reduces their size. It also causes a reduction in the size of the milling media. When this takes place, the fines from the reduction of the milling media will contaminate the dispersion.
Another milling technique for creating dispersions, as shown in U.S. Pat. No. 3,634,252, is sand milling. The technique of sand milling has been used in the prior art for the dispersion of magnetic pigments, not hard particles. When sand milling has been used to disperse fine solids, the prior art has not shown the use of zirconium oxide as a milling media nor a positive attempt to avoid reduction in the size of the hard particles as well as the size of the milling media.
The object of the invention is to provide a method to disperse abrasive resistant hard particles (fine solids) without size reduction.
A further object of the invention is to provide a method to disperse abrasive resistant hard particles without size reduction in a sand mill and using zirconium oxide milling media producing an abrasive resistant magnetic coating.
Another object of the invention is to provide a method to disperse abrasive resistant hard particles in a resin binder matrix to form a dispersion where the dispersion is not contaminated due to a reduction in the size of the milling media.
A further object of the invention is to provide a method to disperse abrasive resistant hard particles in a resin binder matrix to form a dispersion where the sand mill uses non-metallic discs to disperse the abrasive resistant hard particles in producing an abrasive resistant magnetic coating.
A further object of the invention is to provide a method to disperse abrasive resistant hard particles in a resin binder matrix to form a dispersion and provide a finish coating with uniform sized hard particles and magnetic particles for application to a rigid or flexible magnetic recording surface in producing an abrasive resistant magnetic coating.
A still further object of the invention is to provide an abrasive resistant finish coating for application to flexible or rigid recording surfaces that will not damage magnetic heads.
The present invention overcomes these problems as will be set forth in the following.